The present invention generally relates to a technique for exploding a plurality of explosives in a remote control manner, and more particularly relates to a system for blasting a number of detonators in a multiple-step manner by utilizing electromagnetic induction.
There has been developed a remote blasting system using electromagnetic induction. This system has a simple circuit construction and a stable operation and has been used in place of a usual wired blasting system in which an electric blasting apparatus is connected to detonators by means of conductive wires.
The remote blasting system of the kind mentioned above is described in, for instance, U.S. Pat. No. 3,834,310. This blasting system has been advantageously used for blasting explosives under a deep sea where a strong tide runs.
FIG. 1 shows the known blasting system described in the above patent. The blasting system comprises an oscillating unit 1 including an oscillator 2 and a loop antenna 3 connected to the oscillator 2 and a receiving unit 4 which is connected to a detonator 5 inserted into a main explosive 6. In case of effecting the explosion under the sea, the antenna 3 is first arranged on the sea bottom in such a manner that it encloses an area within which the explosion has to be effected. When a switch 2a provided in the oscillator 2 is closed, an A.C. current having a frequency such as 550 Hz is generated from the oscillator 2 and is supplied to the antenna 3. Then an electromagnetic wave is radiated from the antenna 3 toward the receiving unit 4. The receiving unit 4 comprises a coil 7 and a capacitor 8 forming a resonance circuit tuned to the frequency of the A.C. current generated from the oscillator 2. Therefore, due to the electromagnetic wave emitted from the loop antenna 3, in the resonance circuit 7, 8, there is electromagnetically induced an A.C. current. This A.C. current is rectified by a diode 9 and then is charged in an ignition capacitor 10. After the voltage across the ignition capacitor 10 has reached a given threshold value, when the switch 2a is opened, the supply of the A.C. current to the loop antenna 2 is stopped abruptly. This change in the induced A.C. current is detected by a driving circuit 11 and the driving circuit 11 operates to close an electronic switch 12 such as a controlled rectifier. Then the ignition capacitor 10 initiates to discharge through the closed switch 12, contacts 4a, 4b of the receiving unit 4, contacts 5a, 5b of the detonator 5 connected to the contacts 4a, 4b, respectively and a fuse head 13 of the detonator 5. In this manner, the detonator 5 is primarily blasted and then the main explosive 6 is secondarily exploded.
In the known blasting system it is possible to explode a plurality of explosives simultaneously. However, in case of effecting the simultaneous explosion on a large scale, there might be produced serious problems such as vibration of the ground, vibration of the air and shock wave in the water. Nowadays, these problems have to be solved in view of the public pollution. In order to avoid such problems, an amount of explosives which are exploded at a time has to be limited. Then the explosion has to be effected several times and this results in an undesired extension of a term of works.
Under the above circumstances there has been practiced a so-called successive or delay explosion in which a plurality of explosives are exploded not simultaneously but intermittently or successively. Heretofore, in case of effecting the delay explosion there are generally used delay type detonators such as MS detonators and DS detonators. However, in case of effecting the underwater explosion, some detonators which should be blasted later are affected seriously by the shock wave produced by the previous explosion. When a detonator is subjected to the explosion shock wave, there might be produced a dangerous situation that a part of explosives might be remained without being exploded due to the dead pressure phenomenon. Further, due to variation in the delay time of the detonators belonging to the same group which should be exploded simultaneously, one or more detonators in the group might not be exploded due to the dead pressure phenomenon. Particularly, in the underwater explosion, since the pressure of the explosion shock wave is hardly decayed in the water, the influence of the dead pressure phenomenon becomes much more serious.
Further, in case of using the delay type detonators, the delay time is fixed and could not be selected at will. Particularly, it is very difficult or almost impossible to set a relatively long delay time.
In the wired explosion, when a plurality of explosives are separated by sufficient distances and use is made of a special blasting device having special circuits by means of which the delay time can be obtained accurately, it will be possible to effect the successive explosion. However, such a system could not be applied to the underwater explosion, because it is quite difficult to arrange the loop antenna under the sea where the strong tide runs.